[media] V4L2: add a device pointer to struct v4l2_subdev

It is often useful to have simple means to get from a subdevice to the
underlying physical device. This patch adds such a pointer to struct
v4l2_subdev and sets it accordingly in the I2C and SPI cases.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>